Information About NTP

NTP Overview

The Network Time Protocol (NTP) synchronizes the time of day among a set of distributed time servers and clients so that you can correlate events when you receive system logs and other time-specific events from multiple network devices. NTP uses the User Datagram Protocol (UDP) as its transport protocol. All NTP communications use Coordinated Universal Time (UTC).

An NTP server usually receives its time from an authoritative time source, such as a radio clock or an atomic clock attached to a time server, and then distributes this time across the network. NTP is extremely efficient; no more than one packet per minute is necessary to synchronize two machines to within a millisecond of each other.

NTP uses a stratum to describe the distance between a network device and an authoritative time source:

A stratum 1 time server is directly attached to an authoritative time source (such as a radio or atomic clock or a GPS time source).

A stratum 2 NTP server receives its time through NTP from a stratum 1 time server.

Before synchronizing, NTP compares the time reported by several network devices and does not synchronize with one that is significantly different, even if it is a stratum 1. Because Cisco NX-OS cannot connect to a radio or atomic clock and act as a stratum 1 server, we recommend that you use the public NTP servers available on the Internet. If the network is isolated from the Internet, Cisco NX-OS allows you to configure the time as though it were synchronized through NTP, even though it was not.

Note You can create NTP peer relationships to designate the time-serving hosts that you want your network device to consider synchronizing with and to keep accurate time if a server failure occurs.

The time kept on a device is a critical resource, so we strongly recommend that you use the security features of NTP to avoid the accidental or malicious setting of incorrect time. Two mechanisms are available: an access list-based restriction scheme and an encrypted authentication mechanism.

NTP as Time Server

Beginning with Cisco NX-OS Release 5.2, the Cisco NX-OS device can use NTP to distribute time. Other devices can configure it as a time server. You can also configure the device to act as an authoritative NTP server, enabling it to distribute time even when it is not synchronized to an outside time source.

Distributing NTP Using CFS

Cisco Fabric Services (CFS) distributes the local NTP configuration to all Cisco devices in the network. After enabling CFS on your device, a network-wide lock is applied to NTP whenever an NTP configuration is started. After making the NTP configuration changes, you can discard or commit them. In either case, the CFS lock is then released from the NTP application.

Clock Manager

Clocks are resources that need to be shared across different processes and across different VDCs. Multiple time synchronization protocols, such as NTP and Precision Time Protocol (PTP), might be running in the system, and multiple instances of the same protocol might be running in different VDCs.

Beginning with Cisco NX-OS Release 5.2, the clock manager allows you to specify the protocol and a VDC running that protocol to control the various clocks in the system. Once you specify the protocol and VDC, the system clock starts updating. For information on configuring the clock manager, see the Cisco Nexus 7000 Series NX-OS Fundamentals Configuration Guide, Release 5.x.

High Availability

Stateless restarts are supported for NTP. After a reboot or a supervisor switchover, the running configuration is applied. For more information on high availability, see the Cisco Nexus 7000 Series NX-OS High Availability and Redundancy Guide, Release 5.x.

You can configure NTP peers to provide redundancy in case an NTP server fails.

Virtualization Support

If you are running a Cisco NX-OS Release prior to 5.2, up to one instance of NTP is supported on the entire platform. You must configure NTP in the default virtual device context (VDC), and you are automatically placed in the default VDC unless you specify otherwise.

If you are running Cisco NX-OS Release 5.2 or later, multiple instances of NTP are supported, one instance per VDC. By default, Cisco NX-OS places you in the default VDC unless you specifically configure another VDC.

Only one VDC (the default VDC by default) synchronizes the system clock at any given time. The NTP daemon in all other VDCs acts only as an NTP server for the other devices. To change which VDC synchronizes the system clock, use the clock protocol ntp vdc vdc-id command.

NTP recognizes virtual routing and forwarding (VRF) instances. NTP uses the default VRF if you do not configure a specific VRF for the NTP server and NTP peer. See the Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 5.x for more information about VRFs.

For more information about VDCs, see the Cisco Nexus 7000 Series NX-OS Virtual Device Context Configuration Guide, Release 5.x.

Licensing Requirements for NTP

Product

License Requirement

Cisco NX-OS

NTP requires no license. Any feature not included in a license package is bundled with the Cisco NX-OS system images and is provided at no extra charge to you. For a complete explanation of the Cisco NX-OS licensing scheme, see the Cisco NX-OS Licensing Guide.

Prerequisites for NTP

NTP has the following prerequisites:

To configure NTP, you must have connectivity to at least one server that is running NTP.

Guidelines and Limitations

You should have a peer association with another device only when you are sure that your clock is reliable (which means that you are a client of a reliable NTP server).

A peer configured alone takes on the role of a server and should be used as a backup. If you have two servers, you can configure several devices to point to one server and the remaining devices to point to the other server. You can then configure a peer association between these two servers to create a more reliable NTP configuration.

If you have only one server, you should configure all the devices as clients to that server.

You can configure up to 64 NTP entities (servers and peers).

If CFS is disabled for NTP, then NTP does not distribute any configuration and does not accept a distribution from other devices in the network.

After CFS distribution is enabled for NTP, the entry of an NTP configuration command locks the network for NTP configuration until a commit command is entered. During the lock, no changes can be made to the NTP configuration by any other device in the network except the device that initiated the lock.

If you use CFS to distribute NTP, all devices in the network should have the same VRFs configured as you use for NTP.

If you configure NTP in a VRF, ensure that the NTP server and peers can reach each other through the configured VRFs.

You must manually distribute NTP authentication keys on the NTP server and Cisco NX-OS devices across the network.

Use the key keyword to configure a key to be used while communicating with the NTP server. The range for the key-id argument is from 1 to 65535.

Use the maxpoll and minpoll keywords to configure the maximum and minimum intervals in which to poll a peer. The range for the max-poll and min-poll arguments is from 4 to 16 seconds, and the default values are 6 and 4, respectively.

Use the prefer keyword to make this the preferred NTP server for the device.

Use the use-vrf keyword to configure the NTP server to communicate over the specified VRF. The vrf-name argument can be default, management, or any case-sensitive alphanumeric string up to 32 characters.

Note If you configure a key to be used while communicating with the NTP server, make sure that the key exists as a trusted key on the device. For more information on trusted keys, see the “Configuring NTP Authentication” section.

Forms an association with a peer. You can specify multiple peer associations.

Use the key keyword to configure a key to be used while communicating with the NTP peer. The range for the key-id argument is from 1 to 65535.

Use the maxpoll and minpoll keywords to configure the maximum and minimum intervals in which to poll a peer. The range for the max-poll and min-poll arguments is from 4 to 17 seconds, and the default values are 6 and 4, respectively.

Use the prefer keyword to make this the preferred NTP peer for the device.

Use the use-vrf keyword to configure the NTP peer to communicate over the specified VRF. The vrf-name argument can be default, management, or any case-sensitive alphanumeric string up to 32 characters.

Step 4

show ntp peers

Example:

switch(config)# show ntp peers

(Optional) Displays the configured server and peers.

Note A domain name is resolved only when you have a DNS server configured.

Step 5

copy running-config startup-config

Example:

switch(config)# copy running-config startup-config

(Optional) Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Configuring NTP Authentication

You can configure the device to authenticate the time sources to which the local clock is synchronized. When you enable NTP authentication, the device synchronizes to a time source only if the source carries one of the authentication keys specified by the ntp trusted-key command. The device drops any packets that fail the authentication check and prevents them from updating the local clock. NTP authentication is disabled by default.

BEFORE YOU BEGIN

Make sure that you are in the correct VDC. To change the VDC, use the switchto vdc command.

SUMMARY STEPS

1. config t

2. [no] ntp authentication-key number md5 md5-string

3. (Optional) show ntp authentication-keys

4. [no] ntp trusted-key number

5. (Optional) show ntp trusted-keys

6. [no] ntp authenticate

7. (Optional) show ntp authentication-status

8. (Optional) copy running-config startup-config

DETAILED STEPS

Command

Purpose

Step 1

config t

Example:

switch# config t

Enter configuration commands, one per line. End with CNTL/Z.

switch(config)#

Places you in global configuration mode.

Step 2

[no] ntp authentication-key number md5 md5-string

switch(config)# ntp authentication-key 42 md5 aNiceKey

Defines the authentication keys. The device does not synchronize to a time source unless the source has one of these authentication keys and the key number is specified by the ntp trusted-key number command.

The range for authentication keys is from 1 to 65535. Cisco NX-OS Release 5.2(3) and later 5.x releases support up to 15 alphanumeric characters for the MD5 string. Earlier releases support up to 8 alphanumeric characters.

Step 3

show ntp authentication-keys

Example:

switch(config)# show ntp authentication-keys

(Optional) Displays the configured NTP authentication keys.

Step 4

[no] ntp trusted-key number

Example:

switch(config)# ntp trusted-key 42

Specifies one or more keys (defined in Step 2) that a time source must provide in its NTP packets in order for the device to synchronize to it. The range for trusted keys is from 1 to 65535.

This command provides protection against accidentally synchronizing the device to a time source that is not trusted.

Step 5

show ntp trusted-keys

Example:

switch(config)# show ntp trusted-keys

(Optional) Displays the configured NTP trusted keys.

Step 6

[ no ] ntp authenticate

Example:

switch(config)# ntp authenticate

Enables or disables the NTP authentication feature. NTP authentication is disabled by default.

Step 7

show ntp authentication-status

Example:

switch(config)# show ntp authentication-status

(Optional) Displays the status of NTP authentication.

Step 8

copy running-config startup-config

Example:

switch(config)# copy running-config startup-config

(Optional) Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

This example shows how to configure the device to synchronize only to time sources that provide authentication key 42 in their NTP packets:

switch# config t

Enter configuration commands, one per line. End with CNTL/Z.

switch(config)# ntp authentication-key 42 md5 aNiceKey

switch(config)# ntp trusted-key 42

switch(config)# ntp authenticate

switch(config)# copy running-config startup-config

[########################################] 100%

switch(config)#

Configuring NTP Access Restrictions

You can control access to NTP services by using access groups. Specifically, you can specify the types of requests that the device allows and the servers from which it accepts responses.

If you do not configure any access groups, NTP access is granted to all devices. If you configure any access groups, NTP access is granted only to the remote device whose source IP address passes the access list criteria.

BEFORE YOU BEGIN

Make sure that you are in the correct VDC. To change the VDC, use the switchto vdc command.

Creates or removes an access group to control NTP access and applies a basic IP access list.

The access group options are scanned in the following order, from least restrictive to most restrictive. However, if NTP matches a deny ACL rule in a configured peer, ACL processing stops and does not continue to the next access group option.

The peer keyword enables the device to receive time requests and NTP control queries and to synchronize itself to the servers specified in the access list.

The serve keyword enables the device to receive time requests and NTP control queries from the servers specified in the access list but not to synchronize itself to the specified servers.

The serve-only keyword enables the device to receive only time requests from servers specified in the access list.

The query-only keyword enables the device to receive only NTP control queries from the servers specified in the access list.

Step 3

show ntp access-groups

Example:

switch(config)# show ntp access-groups

(Optional) Displays the NTP access group configuration.

Step 4

copy running-config startup-config

Example:

switch(config)# copy running-config startup-config

(Optional) Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

This example shows how to configure the device to allow it to synchronize to a peer from access group “accesslist1”:

switch# config t

switch(config)# ntp access-group peer accesslist1

switch(config)# show ntp access-groups

Access List Type

-----------------------------

accesslist1 Peer

switch(config)# copy running-config startup-config

[########################################] 100%

switch(config)#

Configuring the NTP Source IP Address

NTP sets the source IP address for all NTP packets based on the address of the interface through which the NTP packets are sent. You can configure NTP to use a specific source IP address.

To configure the NTP source IP address, use the following command in global configuration mode:

Command

Purpose

[no] ntp source ip-address

Example :

switch(config)# ntp source 192.0.2.1

Configures the source IP address for all NTP packets. The ip-address can be in IPv4 or IPv6 format.

Configuring the NTP Source Interface

You can configure NTP to use a specific interface.

To configure the NTP source interface, use the following command in global configuration mode:

Command

Purpose

[no] ntp source-interface interface

Example :

switch(config)# ntp source-interface ethernet 2/1

Configures the source interface for all NTP packets. Use the ? keyword to display a list of supported interfaces.

Configuring NTP on a Secondary (Non-Default) VDC

You can configure a non-default VDC to get a timing update from the default VDC and its clients in order to synchronize with it.

BEFORE YOU BEGIN

Use the switchto vdc command to switch to the desired non-default VDC.

SUMMARY STEPS

1. config t

2. feature ntp

3. ntp master

4. (Optional) ntp source-interface interface

5. (Optional) ntp source ip-address

6. (Optional) copy running-config startup-config

DETAILED STEPS

Command

Purpose

Step 1

config t

Example:

switch# config t

Enter configuration commands, one per line. End with CNTL/Z.

switch(config)#

Places you in global configuration mode.

Step 2

feature ntp

Example:

switch(config)# feature ntp

Enables NTP in the non-default VDC.

Step 3

ntp master

Example:

switch(config)# ntp master

Configures the device as an authoritative NTP server.

Step 4

ntp source-interface interface

Example :

switch(config)# ntp source-interface ethernet 2/1

(Optional) Configures the source interface for all NTP packets. Use the ? keyword to display a list of supported interfaces.

Step 5

ntp source ip-address

Example :

switch(config)# ntp source 192.0.2.1

(Optional) Configures the source IP address for all NTP packets. The ip-address can be in IPv4 or IPv6 format.

Step 6

copy running-config startup-config

Example:

switch(config)# copy running-config startup-config

(Optional) Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Configuring NTP Logging

You can configure NTP logging in order to generate system logs with significant NTP events. NTP logging is disabled by default.

BEFORE YOU BEGIN

Make sure that you are in the correct VDC. To change the VDC, use the switchto vdc command.

SUMMARY STEPS

1. config t

2. [no] ntp logging

3. (Optional) show ntp logging-status

4. (Optional) copy running-config startup-config

DETAILED STEPS

Command

Purpose

Step 1

config t

Example:

switch# config t

Enter configuration commands, one per line. End with CNTL/Z.

switch(config)#

Places you in global configuration mode.

Step 2

[no] ntp logging

Example:

switch(config)# ntp logging

Enables or disables system logs to be generated with significant NTP events. NTP logging is disabled by default.

Step 3

show ntp logging-status

Example:

switch(config)# show ntp logging-status

(Optional) Displays the NTP logging configuration status.

Step 4

copy running-config startup-config

Example:

switch(config)# copy running-config startup-config

(Optional) Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

This example shows how to enable NTP logging in order to generate system logs with significant NTP events:

switch# config t

switch(config)# ntp logging

switch(config)# copy running-config startup-config

[########################################] 100%

switch(config)#

Enabling CFS Distribution for NTP

You can enable CFS distribution for NTP in order to distribute the NTP configuration to other CFS-enabled devices.

Enables or disables the device to receive NTP configuration updates that are distributed through CFS.

Step 3

show ntp status

Example:

switch(config)# show ntp status

(Optional) Displays the NTP CFS distribution status.

Step 4

copy running-config startup-config

Example:

switch(config)# copy running-config startup-config

(Optional) Saves the change persistently through reboots and restarts by copying the running configuration to the startup configuration.

Committing NTP Configuration Changes

When you commit the NTP configuration changes, the effective database is overwritten by the configuration changes in the pending database and all the devices in the network receive the same configuration.

To commit the NTP configuration changes, use the following command in global configuration mode:

Command

Purpose

ntp commit

Example :

switch(config)# ntp commit

Distributes the NTP configuration changes to all Cisco NX-OS devices in the network and releases the CFS lock. This command overwrites the effective database with the changes made to the pending database.

Discarding NTP Configuration Changes

After making the configuration changes, you can choose to discard the changes instead of committing them. If you discard the changes, Cisco NX-OS removes the pending database changes and releases the CFS lock.

To discard NTP configuration changes, use the following command in global configuration mode:

Command

Purpose

ntp abort

Example :

switch(config)# ntp abort

Discards the NTP configuration changes in the pending database and releases the CFS lock. Use this command on the device where you started the NTP configuration.

Releasing the CFS Session Lock

If you have performed an NTP configuration and have forgotten to release the lock by either committing or discarding the changes, you or another administrator can release the lock from any device in the network. This action also discards pending database changes.

To release the session lock from any device and discard any pending database changes, use the following command in global configuration mode:

Command

Purpose

clear ntp session

Example :

switch(config)# clear ntp session

Discards the NTP configuration changes in the pending database and releases the CFS lock.

Verifying the NTP Configuration

To display the NTP configuration, perform one of the following tasks:

Command

Purpose

show ntp access-groups

Displays the NTP access group configuration.

show ntp authentication-keys

Displays the configured NTP authentication keys.

show ntp authentication-status

Displays the status of NTP authentication.

show ntp internal

Displays internal NTP information.

show ntp logging-status

Displays the NTP logging status.

show ntp peer-status

Displays the status for all NTP servers and peers.

show ntp peers

Displays all the NTP peers.

show ntp pending

Displays the temporary CFS database for NTP.

show ntp pending-diff

Displays the difference between the pending CFS database and the current NTP configuration.